Pneumatic flotation separation device

ABSTRACT

A separating device ( 10 ) especially for use in a pneumatic flotation system comprises an annular channel ( 18 ), at least one nozzle ( 16 ) for introducing aerated pulp substantially horizontally and tangentially into the annular channel ( 18 ), froth extraction means ( 26 ) for the removal of separated froth from the annular channel ( 18 ), and pulp extraction means ( 24 ) for the removal of separated pulp from the annular channel ( 18 ), the at least one nozzle ( 16 ) being configured so that the aerated pulp introduced into the annular channel ( 18 ) rotates therein with centrifugal forces of between 5 and 20 ms −2 .

The present invention relates to a separation device for use in apneumatic flotation system, especially but not exclusively, for theseparation of slurry.

Slurry comprises a fluid with particles suspended therein, whichtogether form a pulp which is rich in valuable minerals. It is desirableto extract the minerals from the pulp, and this is typically done by aflotation technique in which air bubbles, which retain the minerals, arecreated in the pulp and then separated from it.

A previously proposed method of separation uses mechanical agitation.The pulp is contained in a cascade of tanks, and agitated with impellersand stators, which maintains particle suspension in the pulp, andintroduces air. The air is then dispersed with blowers or byself-aspiration into the impeller mechanism.

An alternative method is that of column flotation, in which pulp isintroduced into the top of a vertical cylindrical vessel, and a streamof bubbles is produced at the bottom of the vessel, and flows upwardlythrough the down-flowing pulp.

A more efficient method is that of pneumatic flotation, in which bubblesare dispersed through the pulp by aeration means prior to theintroduction of the pulp to a separating device. Such devices includethe Jameson Cell and the Bahr Cell. Aerated pulp is introduced intocylindrical or conical tanks, and the flow movement creates someseparation of the bubbles in the form of a froth, which then flows to aperipheral launder. The aerated pulp is introduced typically 2 metresbelow the surface of the pulp already held in the tank, which is likelyto have a diameter of at least 3 metres.

Additionally, the aerated pulp may be introduced into a cylindrical tankin a tangential and horizontal orientation, to create a rotary movementof the pulp, with centrifugal forces generally less than 1 ms⁻². Theamount of time which the pulp is retained in the tank is typicallybetween 2.5 and 3.5 minutes. This duration is required so that low flowrates of the pulp occur, which allows smaller air bubbles, which haverelatively low buoyancy, sufficient time to make their way to the frothon the surface of the pulp. Thus the separation process is relativelyslow, and the quality of the separation can be less than ideal.

An aim of the present invention is to address one or more of theforegoing disadvantages.

Accordingly the present invention is directed to a separating deviceespecially for use in a pneumatic flotation system comprising an annularchannel, at least one nozzle capable of introducing aerated pulp, withpre-established speed, substantially horizontally and tangentially intothe annular channel, froth extraction means for the removal of separatedfroth from the annular channel, and pulp extraction means for theremoval of separated pulp from the annular channel, in which the atleast one nozzle is configured so that the aerated pulp introduced intothe annular channel rotates therein with centrifugal forces of between 5and 20 ms⁻². These large forces act in conjunction with gravitationalforces to rapidly and efficiently separate mineral-carrying air bubblesfrom the fluid and non-floating particulate matter of the pulp, tocreate a improved quality of mineralised froth.

Advantageously the at least one nozzle introduces the aerated pulp tothe annular channel at a level close to below the desired surface levelof the pulp in the annular channel. In this way, the separated pulp,which moves downward under the centrifugal and gravitational forces, isremoved from the path of newly introduced aerated pulp.

In a preferred embodiment on average the time spent by the aerated pulpin the separating device is less than 30 seconds. Less time is hencetaken to separate a given volume of aerated pulp. This is made possibleby the high centrifugal forces, which increase the velocities of thebubbles and particulate matter so that they separate from each othermore quickly.

Advantageously the froth extraction means comprises a froth overflowweir positioned near the inside upper edge of the annular channel. Thegravitational and centrifugal forces create a movement of the bubblestowards the surface of the aerated pulp to form a froth, which is thensimply extracted from the annular channel by overflowing.

Preferably the froth extraction means further comprises a pipe fed bythe froth overflow weir. The overflowing froth is thus collected andpiped away from the separator device.

In a preferred embodiment the pulp extraction means are positioned nearthe base of the annular channel. The means are therefore in the vicinityof the separated pulp.

Preferably the pulp extraction means comprises a pulp overflow weir anda plurality of apertures in the annular channel positioned below the topof the pulp overflow weir. This construction allows for a small volumeof separated pulp to leave the annular channel before the majority ofthe aerated pulp passes over the overflow weir. In this way it ispossible to maintain a near-constant height of pulp within the annularchannel, to compensate for any variations in the rate of aerated pulpintroduction.

Advantageously the pulp extraction means further comprises a tailingspipe fed by the pulp overflow weir and the apertures. The separated pulpextracted via the apertures and via the pulp overflow weir is therebycombined after extraction within a single pipe to be discharged from theseparating device.

Preferably the separating device further comprises a manifold whichdistributes the aerated pulp to the at least one nozzle. The manifoldcan evenly distribute the aerated pulp between a number of nozzles, andcontrol the rate of flow at which the aerated pulp is introduced to theannular channel.

In a preferred embodiment the separating device further comprises anaeration device which supplies the aerated pulp to the manifold.

An example of a separating device made in accordance with the presentinvention will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 shows a cross-section of such a separating device;

FIG. 2 shows a cross-section of the separating device in use; and

FIG. 3 shows a schematic cross-section of an annular channel of theseparating device in use.

FIG. 1 shows a separating device 10. A manifold 12 has a plurality ofpipes 14 leading from it, which each terminate in a nozzle 16. Anannular channel 18 is positioned in a horizontal plane below themanifold. The annular channel 18 has diameter to height ratio of greaterthan three. The pipes 14 connect to the interior of the annular channel18 via the nozzles 16, so that the nozzles 16 lead into the annularchannel 18 in horizontal and tangential positions. The annular channelhas a double internal wall, comprising an outer wall 20 and an innerwall 22. An opening 24 is provided at the base of the inner wall 22 sothat the interior of the annular channel 18 extends to the outer wall20. The inner wall 22 and the outer wall 20 define a verticalcylindrical passage 23.

The inner wall 20 extends in height above the height of the outer wall20. The top edge of the inner wall 20 forms a froth overflow weir 26.Funnel means 28 extend from the weir 26 towards the central axis of theannular channel 18, passing over the top of the outer wall 20, slopingdownwards, and defining a gap 30 between the top of the outer wall 20and the underside of the funnel means 28. A vertical pipe 32 ispositioned below the funnel means 28, and coaxial with the annularchannel 18. The upper edge of the pipe 32 contacts the underside of thefunnel means 28.

A plurality of apertures 34 are provided near the base of the outer wall20, and spaced apart around its circumference. The top of the outer wall20 forms a pulp overflow weir 36.

A tailings pipe 38 is disposed between the outer wall 20 and the pipe 32so that it encompasses the pipe 32, and tapers at its lower end to forman outlet 40 spaced apart from the lower end of the pipe 32. Theinterior of the tailings pipe connects with the interior of the annularchannel via the apertures 34 and the gap 30.

FIG. 2 shows the separating device in use The arrows indicate the flowof the pulp at various stages of the separation process.

Slurry pulp which is to be separated comprises non-floating particlessuspended in fluid, and contains valuable minerals. The pulp is aeratedin an aeration device (not shown) so that it contains many air bubbles,which carry the minerals. The aerated pulp passes into the manifold 12,as indicated by arrow A. The manifold 12 distributes the aerated pulpbetween the pipes 14 so that the pulp flows along the pipes 14 atvelocity. On reaching the nozzles 16, the aerated pulp is introducedhorizontally and tangentially into the annular channel 18. The directionand velocity of introduction cause the aerated pulp to circulate aroundthe annular channel as indicated by arrow B. The circulation velocity isdetermined by the size, number and configuration of the nozzles 16, andthe velocity at which the aerated pulp leaves the nozzles 16. Thenozzles 16 are situated just below the surface of the circulating pulp.

Once circulating, the aerated pulp has two forces acting upon it. Theseare the gravitational force acting downwards and a centrifugal forceacting radially outwards, perpendicular to the axis of rotation. Thecentrifugal force is between 5 and 20 ms⁻². The resultant force arisingfrom the gravitational and centrifugal forces acts downwardly andoutwardly. The direction of the forces is shown schematically in FIG. 3,where arrow C shows the centrifugal force, arrow G shows thegravitational force, and arrow F shows the resultant force.

The resultant force acts to separate the aerated pulp. The bubbles 42,which carry the minerals contained in the slurry, rise upwards againstthe direction of the resultant force, as indicated by arrow D. They riseto the surface 43 of the circulating pulp, in the vicinity of the frothoverflow weir 26. The bubbles 42 combine to form a mineral-containingfroth 46, which under gravity flows over the froth overflow weir 26,down the funnel means 28 and into the pipe 32, as indicated by arrow H.The froth leaves the pipe as indicated by arrow J.

The resultant force acts on the non-floating particles 44 to carry themin the opposite direction to the movement of the bubbles 42, so thatthey settle in the lower part of the annular channel 18, forming aseparated pulp. Continuing separation creates a build-up of separatedpulp, which flows through the opening 24 into the passage 23. A smallportion of the separated pulp comprising predominantly coarse particles,passes through the apertures 34 (arrow K). The remainder of theseparated pulp is forced up the passage 23 by the weight of the newlyformed separated pulp behind it, and flows over the pulp overflow weir36 (arrow L). The two thus-formed flows of separated pulp recombine inthe tailings pipe 38 to form tailings, which are discharged through theoutlet 40 (arrow M).

The size of the apertures 34 is determined so that only a small amountof the separated pulp passes through, with the remainder being forcedover the pulp overflow weir 36. This arrangement acts to regulate therate at which the separated pulp leaves the annular channel 18, so thata roughly constant depth of pulp is maintained within the annularchannel 18. This compensates for any variations in the flow rate fromthe nozzles 16 of the incoming aerated pulp.

The position of the nozzles 16 means that the aerated pulp is introducedinto the annular channel 18 just below the level of the circulatingpulp. This means that the incoming flow does not disturb the previouslyseparated pulp, which has settled to the bottom of the annular channel18.

The relatively high centrifugal force, 5 to 20 ms⁻², creates a resultantforce of sufficient magnitude to act to separate the aerated pulp veryefficiently Even relatively small hydrophilic particles are rapidlycarried downwards and thus prevented from reaching the vicinity of thefroth overflow weir 26. The chance of hydrophilic, non-valuableparticles becoming entrapped in the froth is thus reduced so that thequality of the froth is very good, containing only small amounts ofmisplaced particles.

This rapid and efficient separation process is also highly advantageousas regards the speed of separation. The high resultant force createdacts to separate the bubbles and particles sufficiently quickly that anygiven volume of aerated pulp spends typically less than 30 seconds inthe separating device. This is typically one-sixth of the time requiredin previously proposed pneumatic flotation separation devices.

1. A separating device especially for use in a pneumatic flotationsystem comprising: an annular channel, at least one nozzle capable ofintroducing aerated pulp, with pre-established speed, substantiallyhorizontally and tangentially into the annular channel, froth extractionmeans for the removal of separated froth from the annular channel, andpulp extraction means for the removal of separated pulp from the annularchannel, wherein the at least one nozzle is configured so that theaerated pulp introduced into the annular channel rotates therein withcentrifugal forces of between 5 and 20 ms⁻², wherein the pulp extractionmeans comprises a pulp overflow weir and a plurality of apertures in theannular channel positioned below a top of the pulp overflow weir.
 2. Theseparating device according to claim 1, wherein the at least one nozzleintroduces the aerated pulp to the annular channel at a level closebelow a desired surface level of the pulp in the annular channel.
 3. Theseparating device according to claim 1, wherein on average, a time spentby the aerated pulp in the separating device is less than 30 seconds. 4.The separating device according to claim 1, wherein the froth extractionmeans comprises a froth overflow weir positioned near an inside upperedge of the annular channel.
 5. The separating device according to claim1, wherein the froth extraction means further comprises a pipe fed bythe froth overflow weir.
 6. The separating device according to claim 1,wherein the pulp extraction means are positioned near a base of theannular channel.
 7. The separating device according to claim 1, whereinthe pulp extraction means further comprises a tailings pipe fed by thepulp overflow weir and the apertures.
 8. The separating device accordingto claim 1, further comprising a manifold which distributes the aeratedpulp to the at least one nozzle.
 9. The separating device according toclaim 8, further comprising an aeration device which supplies theaerated pulp to the manifold.
 10. A separating device especially for usein a pneumatic flotation system comprising: an annular channel, at leastone nozzle capable of introducing aerated pulp, with pre-establishedspeed, substantially horizontally and tangentially into the annularchannel, a froth overflow weir disposed in the annular channel, suchthat froth from said aerated pulp will be extracted and removed from theannular channel via a first pipe; a vertical cylindrical passagedisposed adjacent to said annular channel, said vertical cylindricalpassage which receives pulp separated from said annular channel foreventual removal via a second pipe; and a plurality of apertures in saidannular channel, through which a portion of said pulp passes to beremoved from the system via said vertical cylindrical passage.
 11. Aseparating device especially for use in a pneumatic flotation systemcomprising: an annular channel; at least one nozzle capable ofintroducing aerated pulp, with pre-established speed, substantiallyhorizontally and tangentially into the annular channel; froth extractionmeans for the removal of separated froth from the annular channel; andpulp extraction means for the removal of separated pulp from the annularchannel; wherein the at least one nozzle is configured so that theaerated pulp introduced into the annular channel rotates therein withcentrifugal forces of between 5–20 ms⁻², wherein said pulp extractionmeans comprises a vertical cylindrical passage disposed adjacent to saidannular channel, said vertical cylindrical passage which receives pulpseparated from said annular chamber for eventual removal via a secondpipe and a plurality of apertures in said annular channel, through whicha portion of said pulp passes to be removed from the system via saidvertical cylindrical passage.